Various products in modern industry require very precisely processed wafer-type workpieces. These are, for example, highly flat, high-purity annular wafers—afforded narrow tolerances in respect of dimensions—composed of glass or aluminum as substrates for the production of magnetic mass storage devices for computers (hard disks), optical glasses and “flats”, semiconductor wafers for the production of photovoltaic cells, etc. Particularly stringent requirements are made of monocrystalline semiconductor wafers as starting material for functional components appertaining to electronics, microelectronics and microelectromechanics, the production of which will therefore be used hereinafter by way of example for illustrating the invention and the object on which it is based.
Group processing methods are particularly advantageous for the production of semiconductor wafers having a particularly uniform thickness (parallelism of front and rear sides of the semiconductor wafers) and flatness (planarity of front and rear sides), in which group processing methods both sides of the semiconductor wafers are simultaneously processed in material-removing fashion and thus converted into the desired plane-parallel target form, wherein the semiconductor wafers are guided in freely floating fashion and without fixed clamping onto a reference chuck in the processing apparatus. Freely floating double-side group processing methods of this type can be implemented as grinding, lapping and polishing methods.
In this case, both sides of a plurality of semiconductor wafers are simultaneously processed in material-removing fashion between two large ringed-shaped working disks. For this purpose, the semiconductor wafers are individually inserted into receptacle openings in a plurality of thin guide cages. The guide cages are also referred to as carriers and have an outer toothing. The toothing engages into a drive ring (“sun gear”) arranged within the inner circumference of the ring-shaped working disks and a drive ring (“internal gear”) arranged outside the outer circumference of the ring-shaped working disks. As a result of the rotation of working disks, sun gear and internal gear, the carriers and hence the semiconductor wafers describe cycloidal trajectories over the working disks. This arrangement, known as “planetary gearing”, leads to particularly uniform, isotropic and regular processing of the semiconductor wafers.
In the case of lapping, a slurry composed of loose solids having an abrasive action (lapping grain) in a usually oily, glycol-containing or aqueous carrier liquid is supplied to the working gap formed between the working disks, the carriers with the semiconductor wafers moving in said working gap. The working disks contain no substances having an abrasive action in their regions coming into contact with the semiconductor wafers. The material removal is effected by relative movement between working disks and semiconductor wafers under pressure and with the addition of this slurry, also called “lapping slurry”.
In the case of double-side polishing, the working surfaces of the working disks that face the semiconductor wafers are in each case covered with a polishing pad. The working gap, in which the semiconductor wafers move, is thus formed between the polishing pads. Instead of a lapping agent, a polishing agent is fed to said working gap. This is generally an aqueous colloidal dispersion of silica sol having a pH value of between 10 and 13. In this case, the polishing pad contains no abrasive substances that bring about material removal.
In the case of double-side grinding with planetary kinematics, the working surfaces of the working disks that face the workpieces each comprise a working layer having fixedly bonded abrasive substances that engage with the workpieces. A cooling lubricant containing no abrasive substances that bring about mechanical material removal is supplied to the working gap formed between the working layers. The working layer can be a grinding pad which is connected to the working disk by means of adhesive bonding, magnetically, by means of vacuum or in a positively locking manner (e.g. by means of a hook and loop fastener) and can be removed by means of a peeling movement. The abrasive grain fixedly bonded into the grinding pad is preferably diamond, alternatively also silicon carbide (SiC), boron nitride (cubic boron nitride, CBN), boron carbide (B4C), zirconium oxide (ZrO2), aluminum oxide (Al2O3) or mixtures of the materials mentioned. The working layers can also be composed of a multiplicity of stiff grinding bodies containing abrasive substances. Alternatively, the working disks themselves can be embodied as grindstones, i.e. themselves contain abrasive substances, such that no further covering with grinding pads or grinding bodies is required. The cooling lubricant supplied to the working gap is preferably pure water, optionally also with additions of viscosity-changing agents (glycols, hydrocolloids) or agents that chemically support material removal (pH>10). Double-side grinding with planetary kinematics is described for example in DE102007013058A1, an apparatus suitable therefor is described for example in DE19937784A1, suitable grinding pads are disclosed for example in U.S. Pat. No. 5,958,794 and suitable carriers are disclosed for example in DE1020070498A1.
So-called orbital grinding is additionally known, wherein the semiconductor wafers are inserted into a single guide cage, which covers the entire circular (not ring-shaped!) working disk and is driven to effect a gyroscopic movement by means of eccentrics fitted outside the working disks. The method is described for example in US2009/0311863A1.
All of the methods mentioned are intended to lead to semiconductor wafers having a particularly uniform thickness (parallelism of front and rear sides of the semiconductor wafers) and flatness (planarity of front and rear sides). Moreover, the thickness deviations from semiconductor wafer to semiconductor wafer, from batch to batch and between the actual value (actual thickness after processing) and the desired value (target thickness) are intended to be as small as possible. It has been found that comparatively large deviations from batch to batch and between the actual thickness and the target thickness occur in particular in the double-side grinding methods. These deviations can be compensated for only by increased material removal by means of the subsequent steps (double-side polishing) which, on account of the small damage depth of the ground semiconductor wafers, actually manage with very little material removal, such that the process times during double-side polishing are lengthened unnecessarily.